The present invention relates to a covering for photothermal conversion and more particularly applies to the photothermal conversion of solar energy.
It is known that so-called "selective" surfaces are being increasingly used for the photothermal conversion of solar energy, because they make it possible to significantly improve conversion efficiencies. These surfaces are such that they heat by absorbing incident solar radiation in the same way as a black body, but unlike the latter, they only emit very little infrared radiation, so that their heat losses are minimized. Among the different known methods for producing them, that which is most frequently used at present consists of depositing a thin layer of a material absorbing solar radiation on an only slightly emissive layer, i.e. which reflects infrared radiation.
At present there are two main groups of methods used for depositing the thin layer of absorbent materials, namely liquid phase deposition methods (chemical and electrolytic deposits, deposits by immersion, etc.), and vapor phase deposition methods (vacuum evaporation, cathodic sputtering, etc).
The methods of the second group are more difficult to carry out than those of the first group, but nevertheless offer the possibilities of producing composite materials, which would be difficult or even impossible to obtain by using liquid phase deposition methods.
However, the methods of the first and second groups have one point in common, namely with said methods, an attempt is generally made to produce a thin absorbent layer of the "cermet" type, which is a very fine dispersion of a phase having a metallic nature in a matrix having a dielectric nature and in order to increase still further the absorbing properties of the cermet, its composition is made variable in its thickness. Thus, attempts are made to obtain a cermet having a metallic nature at the interface with the infrared radiation reflecting layer and with a dielectric nature at the interface with the ambient medium (air) to obtain a so-called "graded" cermet.
The optical properties of the numerous cermets are now well known and have been widely publicized in the literature. For example, reference can be made to cermets obtained by the reactive cathodic sputtering of a stainless steel target in a residual atmosphere of argon and acetylene. Cermets of this type are, for example, envisaged in U.S. Pat. No. 4,309,261 and in a communication entitled "In line production system for sputter deposition of graded index solar absorbing films", by D. R. McKenzie et al., 8th International Vacuum Congress, Cannes, September 1980.
Apart from the fact that reactive cathodic sputtering is much more difficult to control than non-reactive cathodic sputtering due, inter alia, to the pressure gradient of the reactive gas which has to be maintained in the sputtering chamber, the layers of the type referred to in the aforementioned paragraph and obtained by reactive cathodic sputtering, contain a large proportion of hydrogen, as has been stated in the article entitled "Properties of hydrogenated carbon films produced by reactive magnetron sputtering", by D. R. McKenzie et al., published in Solar Energy Materials, 6, 1981, pp. 97-106.
The presence of hydrogen in the deposited layer is prejudicial to the production of a vacuum transducer for photothermal conversion, because it makes the degassing operations which have to be carried out during its production more difficult. Moreover, the desorption of this hydrogen during heat treatment seems to be one of the most important reasons for the deterioration in the optical properties of graded cermets produced from stainless steel and carbon.
Coverings for photothermal conversion are also known, which are obtained by a method consisting of depositing a graphite layer on an infrared radiation reflecting layer,, the latter being itself deposited on a substrate and which can be of copper, silver, nickel or titanium. This method is described in the article entitled "Effect of substrate on graphite and other solar selective surfaces", by D. R. McKenzie, published in Applied Optics, vol. 17, no. 12, 1978, pp. 1984 to 1988. It is much simpler than the reactive cathodic sputtering method and can therefore be much more easily controlled, because it only involves the superimposing of two elementary homogeneous layers. Moreover, these layers are free from prejudicial foreign atoms, such as hydrogen atoms, due to the procedure used for depositing the graphite layer, namely vacuum evaporation by an electron gun.
Unfortunately, the graphite layers obtained by this method are not sufficiently absorbent to be suitable for industrial applications. Thus, their solar absorption factor is only 0.70 for graphite deposited on copper and 0.80 for graphite deposited on titanium.